Because of their ability to target virtually any molecule with exquisite specificity, monoclonal antibodies have the potential to become one of the main therapeutic agents of the future. Though this potential was recognized several years ago, however the first attempts to fulfill the potential were disappointing because monoclonal antibodies used in therapy elicit a strong immune response in patients (Schroff, 1985 Cancer. Res. 45:879-85, Shawler. J Immunol 1985 135:1530-5), even at low doses (Dillman, Cancer Biother. 1994 9:17-28). Scientists predict that human antibodies would not cause such adverse immune responses. However, no suitable methods exist for producing human monoclonal antibodies. Alternative technologies to make human antibodies using, for example, phage display and transgenic animals have been developed more recently but are not widely used for therapeutic purposes.
The immunogenicity of antibodies depends on many factors, including the method of administration, the number of injections, the dosage, the nature of the conjugation, the specific fragment utilized, the state of aggregation and the nature of the antigen (e.g., Kuus-Reichel, Clin. Diagn. Lab. Immunol. 1994 1:365-72). Many or most of these factors can be manipulated in order to decrease an immune response. However, if the original antibody sequence is recognized as “dangerous” or “foreign”, the chances are that sooner or later a strong immune response will prevent the use of that antibody in therapy.
In order to decrease these responses, efforts have been made to replace as much as possible of the non-human sequence of an antibody with human sequences using recombinant DNA technology. Towards this end, chimeric antibodies containing human antibody light chain and heavy chain constant domains that are joined to mouse antibody variable light chain and heavy chain domains have been employed. Chimeric antibodies still contain a large number of non-human amino acid sequences in the variable regions and, as such, a significant immune response may be mounted against such antibodies. CDR grafting is another humanization technique in which the antigen binding portions or “complementarity determining regions” (CDRs) of monoclonal antibodies are grafted by recombinant DNA technologies into the DNA sequences encoding the framework (i.e. the non-CDR region) of human antibody heavy and light chains. One technical problem of CDR grafted antibodies is that they usually show considerable decreased affinity. To restore increase the affinity of CDR grafted antibodies, certain original key framework residues (e.g., residues that are thought to be involved in determining the conformation of the CDRs) are reintroduced into the CDR grafted antibody. Using a different humanization approach, Roguska devised a “resurfacing” strategy for mouse antibodies where only exposed residues that are different to exposed residues of a human antibody are substituted.
However, although antibodies humanized by the above methods can show reduced immunogenicity in human patients (Moreland, Arthritis Rheum 1993 36:307-18) many humanized antibodies are still highly immunogenic to a large proportion of patients. This is thought to be because the CDRs themselves are immunogenic (Ritter, Cancer Res 2001 61:6851-9; Welt, Clin Cancer Res 2003 9:1338-46).
All of the methods described above require that the CDR regions of the non-human antibody remain unchanged during the humanization process in order to maintain antibody specificity and affinity. However, since non-human CDR regions are themselves immunogenic in humans, methods for humanizing the CDR regions of a non-human antibody without significantly reducing the binding activity of the antibody are highly desirable. The identification of suitable methods for humanizing the CDR regions of a non-human antibody has been a daunting, if not impossible, task for the medical and research community.
Accordingly, there is an ongoing need for improved methods for making non-human antibodies that are less immunogenic in humans and other mammalian hosts. In particular, there is a need for humanization methods that reduce the immunogenicity of CDR regions of a non-human antibody in humans.